The present invention concerns polyolefin compositions having improved tenacity, while retaining softness and good mechanical properties, which can be advantageously used in flat sheet material, such as single ply roofing membranes for covering industrial and commercial flat roofs, and in other roofing applications. These compositions may be obtained by a sequential polymerization process.
Traditionally, the building industry has utilized conventional built-up asphalt or fiber glass roofing as a preferred material in roofing construction. More recently, however, membrane roofing materials have displaced the conventional materials as a preferred material due to their cold cracking resistance, ease of installation, and overall improved and increased leak protection over time. Further, the membrane systems are easier and safer to install and are therefore more desirable to the contractor as well as the consumer.
Two membrane types are utilized in this field: thermoplastic and elastomeric.
Thermoplastic membranes, such as those formed from polyvinyl chloride (PVC), chlorinated polyethylene (CPE), chlorosulfonated polyethylene and the like, can be heat sealed or solvent welded to provide dependable seals of higher strength; however, these membranes also have serious disadvantages. For example, the thermoplastic material must be plasticized to provide the flexibility necessary for a roofing membrane, and plasticizers are quite expensive and leach out of the membrane over time thus resulting in the loss of flexibility, embrittlement and decreased cold crack resistance of the membrane. Further, the capacity of the thermoplastic materials to accept fillers is limited.
Moreover, there is a need for a PVC sheeting alternative, in view of the trend toward a chlorine-free environment.
Elastomeric membranes, such as vulcanized ethylene/propylene/diene terpolymers EPDM rubbers, has been rapidly gaining acceptance, because of outstanding weathering resistance and flexibility. This material normally is prepared by vulcanizing the composition in the presence of sulfur or sulfur containing compounds.
However, these membranes are difficult to seal, due to the lack of adhesion of EPDM, especially cured EPDM, to itself, and require an adhesive for seaming the membrane in order to provide a leak-free, continuous roofing cover. These adhesives add a significant material cost and are difficult and time-consuming to apply. Further, the adhesives often weaken over time, causing delamination of the membranes and subsequent leaks in the roofing cover. Elastomeric membranes also require an additional costly curing step.
In order to avoid the curing procedure, in the state of the art it was proposed to use sheeting materials for roofing applications comprising an EPDM in combination with highly crystalline thermoplasticity promoters, such as high density polyethylene (HDPE), low density polyethylene (LDPE) and other similar olefin type polymers. For instance, U.S. Pat. No. 5,256,228 discloses a heat seamable sheet material for roofing, prepared from an uncured polymeric composition which comprises 100 parts by weight of a polymer blend comprising from 50 to 90 parts by weight of polyolefins having up to 2 percent by weight crystallinity, and from 10 to 50 parts by weight of a highly crystalline thermoplasticity promoter such as HDPE or LDPE; 50-250 parts by weight of a filler; and 20-150 parts by weight of a processing material.
Nevertheless, the presence of crystalline polyethylene resins have many disadvantages. For instance, higher crystallinity levels result in less thermal dimensional stability, expansion or contraction over large temperature ranging observed in the field; this may result in bucking or stress on the sheet. Moreover, the lower melting characteristics of polyethylene result in limitations for applications at elevated temperatures, such as in dark colored sheet. Finally, polyethylene has a narrow melting point range, which results in a narrow heat welding process window.
Polyolefin compositions having good mechanical properties have been used in many application fields, due to the characteristics which are typical of polyolefins (such as chemical inertia, mechanical properties and nontoxicity); moreover, these compositions show outstanding cost/performance ratios. In the state of the art, these compositions have been obtained by way of sequential copolymerization of propylene, optionally containing minor quantities of olefin comonomers, and then ethylene/propylene or ethylene/alpha-olefin mixtures. Catalysts based on halogenated titanium compounds supported on magnesium chloride are commonly used for this purpose.
For instance, U.S. Pat. No. 5,286,564, in the name of the same Applicants, describes flexible polyolefin compositions comprising, in parts by weight:
A) 10-50 parts of an isotactic propylene homopolymer or copolymer;
B) 5-20 parts of an ethylene copolymer, insoluble in xylene at room temperature; and
C) 40-80 parts of an ethylene/propylene copolymer containing less than 40% by weight of ethylene and being soluble in xylene at room temperature; the intrinsic viscosity of said copolymer is preferably from 1.7 to 3 dl/g.
Said compositions have flexural modulus values of less than 150 MPa and Shore D hardness between 20 and 35. Although these mechanical properties are advantageous for certain applications, the compositions prepared in Examples 1-5 show values of Tensile Strength at Break ranging from 13.8 to 17.3 MPa; for many applications, such as in roofing sheets, these polymers do not show a satisfactory tenacity, which is a property strictly related to tensile strength at break and to elongation at break properties. Tenacity is fundamental in roofing applications, wherein there is the tendency of roofing to break under the action of the wind.
As it is well known in the art, the strength and flexibility of thermoplastic polyolefin compositions may be enhanced decreasing the rubber content; nevertheless, to the increase in strength is usually associated an increase in stiffness that causes various drawbacks. For instance, the increase in stiffness of a flexible membrane limits the installation of the membrane around corner, chimneys, vents etc., where the membrane must be flexible to bend around the obstruction.
The European Patent Application No. 1202876.7, in the name of the same Applicants, describes a polyolefin composition comprising:
(A) from 8 to 25% by weight of a crystalline propylene homopolymer or copolymer; and
(B) from 75 to 92% by weight of an elastomeric fraction comprising a first elastomeric copolymer of propylene with 15-32% wt ethylene, the intrinsic viscosity of the xylene soluble fraction ranging from 3.0 to 5.0 dl/g; and a second elastomeric copolymer of propylene with more than 32% up to 45% wt. of ethylene, the intrinsic viscosity of the xylene soluble fraction ranging from 4.0 to 6.5 dl/g.
These polyolefin compositions have flexural modulus values lower than 60 MPa and very good hardness values (Shore A hardness lower than 90); nevertheless, they exhibit low tenacity values. In fact, the polyolefin compositions prepared in Examples 1-3 of the above-mentioned application show values of Tensile Strength at Break equal to 5.5, 11.7 and 11.2 MPa respectively. As reported above, such values are not fully satisfactory for roof sheeting, since the increase in flexibility is associated to an enhanced stiffniess.
A strong need therefore exists for softer materials which are easier to install, and at the same time possess a satisfactory tenacity, strength, and tear and puncture resistance. In other words, it is felt the need for polyolefin compositions which exhibit a good balance of flexibility over a variable temperature range, and tenacity, as well as good mechanical properties; moreover, these compositions must be heat-sealable and oil-resistant and must not undergo degradation when exposed to the elements.
The present invention concerns a polyolefin composition comprising:
(A) from about 15 to about 40% by weight of a crystalline copolymer of propylene with at least one alpha-olefin of formula H2Cxe2x95x90CHR1, where R1 is H or a C2xe2x88x928 linear or branched alkyl, containing at least about 90% by weight of propylene, and having solubility in xylene at room temperature lower than about 15% by weight;
(B) from about 60 to about 85% by weight of an elastomeric fraction comprising:
(1) a copolymer of propylene with ethylene, optionally containing about 0.5 to 5% by weight of a diene, containing from about 20 to about 35% by weight ethylene, and having solubility in xylene at room temperature greater than about 70% by weight, the intrinsic viscosity of the xylene soluble fraction being higher than about 3.0 dl/g up to about 6.0 dl/g; and
(2) a copolymer of ethylene with at least one alpha-olefin of formula H2Cxe2x95x90CHR2, where R2 is a C2xe2x88x928 linear or branched alkyl, optionally containing about 0.5 to 5% by weight of a diene, containing about 15% to 40% by weight alpha-olefin, and having solubility in xylene at room temperature greater than about 25% by weight, the intrinsic viscosity of the xylene soluble fraction ranging from about 0.5 to about 5.0 dl/g;
the (1)/(2) weight ratio ranging from about 1:5 to about 5:1.
The present invention further concerns a process for the preparation of the polyolefin composition reported above, comprising at least three sequential polymerization stages, with each subsequent polymerization being conducted in the presence of the polymeric material formed in the immediately preceding polymerization reaction, wherein the crystalline copolymer (A) is prepared in a first stage, and the elastomeric fraction (B) is prepared in at least two subsequent stages. According to a preferred embodiment, all the polymerization stages are carried out in the presence of a catalyst comprising a trialkylaluminum compound, optionally an electron donor, and a solid catalyst component comprising a halide, or halogen-alcoholate of Ti, and an electron donor compound supported on anhydrous magnesium chloride, said solid catalyst component having a surface area (measured by BET) of less than about 200 m2/g, and a porosity (measured by BET) higher than about 0.2 ml/g.
The polyolefin compositions of the present invention show a significant improvement in strength and toughness, without sacrificing stiffness; on the opposite, applicants have surprisingly found that the stiffness of the compositions of the invention is reduced with respect to prior art compositions containing lower amounts of rubber content. More specifically, the compositions of the invention are endowed with high tenacity, as evidenced by the values of tensile strength at break and elongation at break much higher than the ones of the prior art thermoplastic resins used in flexible roofing membranes, at the same time showing a remarkable reduction in stiffness.
The compositions of the invention exhibit a very good balance of flexibility and mechanical properties, and in particular of flexural modulus values and Shore D hardness, at the same time retaining good elastomeric properties. These soft compositions, easier to install even around corners and chimneys, show better strength and tear resistance, as well as improved wind uplift and puncture resistance; moreover, they are endowed with long term weldability.
The polyolefin compositions of the present invention comprise from about 15 to about 40% by weight, preferably from about 15 to about 35%, and even more preferably from about 20 to about 30%, of a crystalline copolymer of propylene (A), and from about 60 to about 85% by weight, preferably from about 65 to about 85%, and even more preferably from about 70 to about 80% by weight of an elastomeric fraction (B).
In the present description, by xe2x80x9croom temperaturexe2x80x9d is meant approximately 25xc2x0 C.
The crystalline copolymer (A) of the compositions of the invention is a copolymer of propylene with an alpha-olefin H2Cxe2x95x90CHR1, where R1 is H or a C2xe2x88x928 linear or branched alkyl, containing at least about 90% by weight of propylene, preferably at least about 95% propylene. This copolymer has solubility in xylene at room temperature lower than about 15% by weight, preferably lower than about 10%, and even more preferably lower than about 8%. Said alpha-olefin is preferably ethylene, 1-butene, 1-pentene, 4-methylpentene, 1-hexene, 1-octene or combinations thereof, and even more preferably it is ethylene.
The elastomeric fraction (B) of the polyolefin compositions of the invention comprises a copolymer of propylene with ethylene, and a copolymer of ethylene with an alpha-olefin H2Cxe2x95x90CHR2, where R2 is a linear or branched C2xe2x88x928 alkyl. By xe2x80x9celastomenrcxe2x80x9d is meant herein a polymer having low cristallinity or amorphous, and having a solubility in xylene at room temperature greater than about 40% by weight.
The copolymer (1) of propylene with ethylene contains from about 20 to about 35% by weight of ethylene, preferably from about 25 to about 30%, and has a solubility in xylene at room temperature greater than about 70% by weight, preferably greater than about 75%; the intrinsic viscosity of the xylene soluble fraction ranges from about 3.0 to about 6.0 dl/g, more preferably from about 3.5 to about 5.0 dl/g, and even more preferably from about 4.0 to about 4.5 dl/g.
The copolymer (2) is a copolymer of ethylene with at least one alpha olefin of formula H2Cxe2x95x90CHR2, where R2 is a C2xe2x88x928 linear or branched alkyl, optionally containing about 0.5 to 5% by weight of a diene; said alpha-olefin is preferably 1-butene, 1-hexene or 1-octene, and even more preferably is 1-butene. The alpha-olefin content ranges from about 15% to about 40% by weight, and preferably from about 20 to about 35%. The copolymer (2) has solubility in xylene at room temperature greater than about 25% by weight, preferably greater than about 30%, and the intrinsic viscosity of the xylene soluble fraction ranges from about 0.5 to about 5.0 dl/g, preferably from about 1.0 to about 4.5 dl/g.
As previously reported, the copolymers (1) and (2) of the elastomeric fraction (B) may contain a diene, conjugated or not, such as butadiene, 1,4-hexadiene, 1,5-hexadiene and ethylidene-norbornene-1. The diene, when present, is contained in an amount of from about 0.5 to about 5% by weight, with respect to the weight of the fraction (B).
The weight ratio of the elastomeric copolymers (1)/(2) ranges from about 1:5 to about 5:1, and preferably from about 1:2 to about 2:1.
The polyolefin composition of the invention, preferably prepared by sequential polymerization in at least three stages, shows high tenacity, and in particular has tensile strength at break 25 MPa (ASTM D 882, on a 100 xcexcm film), more preferably xe2x89xa728 MPa; elongation at break 700 MPa (ASTM D 882, on a 100 xcexcm film), more preferably xe2x89xa7750 MPa; and toughness 150 MPa (ASTM D 638, on a 1 mm sheet), more preferably 170 MPa.
Moreover, said composition has a flexural modulus 130 MPa, preferably 100 MPa, and more preferably 80 MPa; Shore D hardness 40xc2x0 S, and preferably ranging from about 25 to about 35; and MFR  less than 1.0 g/10 min, preferably  less than 0.8 g/10 min, and even more preferably  less than 0.6 g/10 min.
According to a preferred embodiment of the invention, the polyolefin composition is in the form of spherical particles having an average diameter of about 250 to about 7,000 microns, a flowability of less than about 30 seconds and a bulk density (compacted) greater than about 0.4 g/ml.
The polyolefin composition of the invention may be prepared by sequential polymerization in at least three stages; according to a preferred embodiment, the sequential polymerization is carried out in the presence of a catalyst comprising a trialkylaluminum compound, optionally an electron donor, and a solid catalyst component comprising a halide or halogen-alcoholate of Ti, and an electron-donor compound supported on anhydrous magnesium chloride.
The present invention is further directed to a process for the preparation of the polyolefin compositions as reported above, said process comprising at least three sequential polymerization stages with each subsequent polymerization being conducted in the presence of the polymeric material formed in the immediately preceding polymerization reaction, wherein the crystalline copolymer (A) is prepared in at least one first stage, and the elastomeric fraction (B) is prepared in at least two sequential stages. The polymerization stages may be carried out in the presence of a Ziegler-Natta and/or a metallocene catalyst.
Suitable Ziegler-Natta catalysts are described in EP-A-45 977 and U.S. Pat. No. 4,399,054, incorporated herein by reference; other examples can be found in U.S. Pat. No. 4,472,524, incorporated herein by reference.
The solid catalyst components used in said catalysts comprise, as electron-donors (internal donors), compounds selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids.
Particularly suitable electron-donor compounds are phthalic acid esters, such as diusobutyl, dioctyl, diphenyl and benzylbutyl phthalate.
Other suitable electron-donors are 1,3-diethers of formula: 
wherein RI and RII, the same or different from each other, are C1-C18 alkyl, C3-C18 cycloalkyl or C7-C18 aryl radicals; RIII and RIV, the same or different from each other, are C1-C4 alkyl radicals; or are the 1,3-diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6 or 7 carbon atoms and containing two or three unsaturations.
Ethers of this type are described in EP-A-361 493 and EP-A-728 769.
Representative examples of said dieters are 2-methyl-2-isopropyl-1,3-dimethoxypropane, 2,2-diisobutyl-1,3-dimethoxypropane, 2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane, 2-isopropyl-2-isoamyl-1,3-dimethoxypropane, and 9,9-bis(methoxymethyl)fluorene.
The preparation of the above mentioned catalyst components is carried out according to various methods.
According to a preferred method of preparation, a MgCl2.nROH adduct (in particular in the form of spheroidal particles) wherein n is generally from 1 to 3 and ROH is ethanol, butanol or isobutanol, is reacted with an excess of TiCl4 containing the electron-donor compound. The reaction temperature is generally from 80 to 120xc2x0 C. The solid is then isolated and reacted once more with TiCl4, in the presence or absence of the electron-donor compound, after which it is separated and washed with aliquots of a hydrocarbon until all chlorine ions have disappeared.
In the solid catalyst component the titanium compound, expressed as Ti, is generally present in an amount from about 0.5 to about 10% by weight. The quantity of electron-donor compound which remains fixed on the solid catalyst component generally is about 5 to 20% by moles with respect to the magnesium dihalide.
The titanium compounds which can be used in the preparation of the solid catalyst component are the halides and the halogen alcoholates of titanium. Titanium tetrachloride is the preferred compound.
The reactions described above result in the formation of a magnesium halide in active form. Other reactions are known in the literature, which cause the formation of magnesium halide in active form starting from magnesium compounds other than halides, such as magnesium carboxylates.
The Al-alkyl compounds used as co-catalysts comprise Al-trialkyls, such as Al-triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, SO4 or SO3 groups. The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from about 1 to about 1000.
The electron-donor compounds that can be used as external donors include aromatic acid esters such as alkyl benzoates, and in particular silicon compounds containing at least one Sixe2x80x94OR bond, where R is a hydrocarbon radical. Examples of silicon compounds are (tert-butyl)2Si(OCH3)2, (cyclohexyl)(methyl)Si(OCH3)2, (phenyl)2Si(OCH3)2 and (cyclopentyl)2Si(OCH3)2. 1,3-diethers having the formulae described above can also be used advantageously. If the internal donor is one of these dieters, the external donors can be omitted.
The solid catalyst component have preferably a surface area (measured by BET) of less than about 200 m2/g, and more preferably ranging from about 80 to about 170 m2/g, and a porosity (measured by BET) preferably greater than about 0.2 ml/g, and more preferably from about 0.25 to about 0.5 ml/g.
The catalysts may be precontacted with small quantities of olefin (prepolymerization), maintaining the catalyst in suspension in a hydrocarbon solvent, and polymerizing at temperatures from ambient to 60xc2x0 C., thus producing a quantity of polymer from about 0.5 to about 3 times the weight of the catalyst. The operation can also take place in liquid monomer, producing, in this case, a quantity of polymer 1000 times the weight of the catalyst.
By using the above-mentioned catalysts, the polyolefin compositions are obtained in spheroidal particle form, the particles having an average diameter from about 250 to about 7,000 microns, a flowability of less than about 30 seconds and a bulk density (compacted) greater than about 0.4 g/ml.
Other catalysts that may be used in the process according to the present invention are metallocene-type catalysts, as described in EP-A-0 129 368 and U.S. Pat. No. 5,324,800, incorporated herein by reference; particularly advantageous are bridged bis-indenyl metallocenes, for instance as described in EP-A-0 485 823 and U.S. Pat. No. 5,145,819, incorporated herein by reference. Another class of suitable catalysts are the so-called constrained geometry catalysts, as described in EP-A-0 416 815, EP-A-0 420 436, EP-A-0 671 404, EP-A-0 643 066 and WO 91/04257. These metallocene compounds may be used to produce the copolymers (1) and (2) of the elastomeric fraction (B).
According to a preferred embodiment, the polymerization process of the invention comprises three stages, all carried out in the presence of Ziegler-Natta catalysts, where: in the first stage the relevant monomer(s) are polymerized to form the crystalline copolymer (A); in the second stage a mixture of propylene and ethylene, and optionally a diene are polymerized to form the copolymer (B)(1); and in the third stage a mixture of ethylene and an alpha-olefin H2Cxe2x95x90CHR2, and optionally a diene, are polymerized to form the copolymer (B)(2).
The polymerization stages may occur in liquid phase, in gas phase or liquid-gas phase. Preferably, the polymerization of the crystalline copolymer fraction (A) is carried out in liquid monomer (e.g. using liquid propylene as diluent), while the copolymerization stages of the copolymers (B)(1) and (B)(2) are carried out in gas phase, without intermediate stages except for the partial degassing of the propylene. According to a most preferred embodiment, all the three sequential polymerization stages are carried out in gas phase.
The reaction temperature in the polymerization stage for the preparation of the crystalline copolymer (A) and in the preparation of the copolymers (B)(1) and (B)(2) can be the same or different, and is preferably from about 40xc2x0 C. to about 90xc2x0 C.; more preferably, the reaction temperature ranges from about 50 to about 80xc2x0 C. in the preparation of the fraction (A), and from about 40 to about 80xc2x0 C. for the preparation of components (B)(1) and (B)(2).
The pressure of the polymerization stage to prepare the fraction (A), if carried out in liquid monomer, is the one which competes with the vapor pressure of the liquid propylene at the operating temperature used, and is possibly modified by the vapor pressure of the small quantity of inert diluent used to feed the catalyst mixture, and the overpressure of the monomers and the hydrogen optionally used as molecular weight regulator.
The polymerization pressure preferably ranges from about 33 to about 43 bar, if done in liquid phase, and from about 5 to about 30 bar if done in gas phase. The residence times of the three stages depend on the desired ratio between the fractions, and can range from about 15 minutes to 8 hours. Conventional molecular weight regulators known in the art, such as chain transfer agents (e.g. hydrogen or ZnEt2), may be used.
The compositions of the invention may be used in combination with other elastomeric polymers, such as ethylene/propylene copolymers (EPR), ethylene/ propylene/diene terpolymers (EPDM), copolymers of ethylene with C4-C12 alpha-olefins (e.g. ethylene/octene-1 copolymers, such as the ones commercialized under the name Engage(copyright)) and mixtures thereof. Such elastomeric polymers may be present in an amount of 5 to 80% by weight on the total composition.
In addition to its polymer components, the composition of the present invention can be blended with common additives, such as reinforcing and non-reinforcing fillers, pigments, nucleating agents, extension oils, mineral fillers, flame retardants (e.g. aluminum trihydrate), antioxidants, UV resistants (e.g. titanium dioxide), UV stabilizers, lubricants (e.g., oleamide), antiblocking agents, antistatic agents, waxes, coupling agents for fillers and other processing aids known in the polymer compounding art. The pigments and other additives may comprise up to about 50 weight percent of the total composition based on polymer components plus additives; preferably pigments and fillers comprise above about 0 to about 30 weight percent of the total composition.
In view of the properties described above, the polyolefin compositions of the present invention are valuable in the production of roofing membranes for covering industrial and commercial flat roofs, and in other roofing applications. In fact, they possess a high tenacity, while retaining softness and good mechanical properties.
Moreover, the roofing membranes obtained with the polyolefin compositions of the invention are weatherproof, inexpensive to manufacture, and durable, having an average life far greater than that of current commercial roofing shingles which are made using fiberglass or asphalt.
Roofing membranes formed from the polyolefin compositions of the present invention may be produced by any method conventionally used for producing roofing membranes from filled polymer compositions. For example, the membranes may be formed by conventional milling, calendering or extrusion techniques. Other methods, including spray coating and roller die forming, may be used. According to a preferred embodiment, the polymeric compositions of the invention are sheeted to thickness ranging from about 5 to about 200 mils. The sheeting can be cut to desired length and width dimensions at this time.
Roofing membranes formed from the compositions of the present invention may optionally be scrim reinforced. Besides roofing membranes, the polyolefin compositions of this invention may be used to produce underlay materials, used as replacements for roofing felt.
Roofing membranes can be applied in the same manner and circumstances as conventional asphaltic or fiberglass membranes. They have light weight, providing potential cost savings in the design of structural roof supports, as well as ease of application. They provide outstanding strength and durability, together with excellent temperature stability, weatherability and resiliency.
A method for covering a roof comprises the steps of applying sheets of the polymeric material of the invention to the roof being covered; overlapping adjacent edges of the layers; heating the overlapped areas to slightly above the softening point of the sheet material and seaming the overlapped areas using heat and under sufficient pressure to provide an acceptable seam strength. The polymeric compositions have sufficient self-adhesion without the use of an adhesive.
The following analytical methods have been used to determine the properties reported in the detailed description and in the examples.
Determination of Xylene Solubility at Room Temperature (% by weight):
2.5 g of polymer were dissolved in 250 ml of xylene, at 135xc2x0 C., under agitation. After 20 minutes, the solution was cooled to 25xc2x0 C. under stirring, and then it was allowed to settle for 30 minutes.
The precipitate was filtered with filter paper; the solution was evaporated under a nitrogen current, and the residue dried under vacuum at 80xc2x0 C. until reached constant weight. The weight percentage of polymer soluble in xylene at room temperature was then calculated. The percent by weight of polymer insoluble in xylene at room temperature was considered the isotactic index of the polymer. This value corresponded substantially to the isotactic index determined by extraction with boiling n-heptane, which by definition constitutes the isotactic index of polypropylene.
Unless otherwise specified, the samples to be subjected to the various physical-mechanical analyses were molded by use of a Negri and Bossi injection press 90, after stabilizing the sample with IRGANOX R 1010 hindered phenolic stabilizer (0.05% by weight), and IRGAFOS 168 phosphite stabilizer (0.1% by weight), and pelletizing the sample with a twin-screw Berstorff extruder (barrel diameter 25 mm) at 210xc2x0 C.
The conditions were as follows:
temperature of the melt: 220xc2x0 C.;
temperature of the mold: 60xc2x0 C.;
injection time: 9 sec;
cooling time: 15 sec.
The dimensions of the plaques for the tests were 127xc3x97127xc3x972.5 mm. From these plaques, C-type dumbbells were cut and submitted to tensile strength tests with a head speed of 500 mm/min. Also the specimens for the flexural modulus and hardness Shore D were cut from these plaques. All the specimens were cut parallel to the advancement front and consequently perpendicular to the flow direction.